The crown is dead . What you are seeing is the currents moving it . You will want to watch to see that he is not being picked on by fish or crabs . Some times they just molt thier crown to regrow it as it becomes old and tattered or something may have irritated it ... It is normal for them to decrown but it will grow back when we had ours he molted his crown every 6months or so .There are many reasons to why they would decrown but it will grow back if he has clean water and nothing stressing him
Watch to see that you dont see him out ( they are ugly and look like a milipede under water and are usually dark brown ) Once they leave the tube it is less than 2% chance they will rebuild a tube and survive ... Look at the end of the tube does it look closed ? That is one of their defences it keeps prey out such as crab or shrimp claws .... It may not always look closed if he is eating but in a few weeks he should reemerge with a new (but smaller) crown ...